David gave a demo of his soon to be released Retrospective Tool (thanks David!)
We discussed which exercises would be suitable for remote/distributed retrospectives, the conclusion was that almost all exercises can be used in such a setting. It depends however on the skills of the facilitator who should be capable enough to manage a retrospective that is ongoing in multiple locations.
Luis Gonçalves wrote a blog post [http://sumo.ly/3x0r 21 Mandatory Tools for Distributed Agile Retrospectives] which provides information on tools. Ben Linders maintains a [http://www.benlinders.com/exercises/ Retrospectives Exercises] page which also contains exercises that are suitable for remote/distributed retrospectives.